People ask me all the time whether they really have to rehydrate dry yeast before pitching, and the honest answer now is: usually not. The producers used to tell everyone to rehydrate, and then they stopped recommending it for homebrewers. The reason is simple: today's dry yeast comes through drying and rehydration far better than it once did. Sprinkle a fresh packet across a typical wort and the great majority of the cells revive; survival can run north of 90 percent. Years ago you might have lost half the packet doing exactly the same thing. So for a lot of beers, sprinkling straight onto the wort is perfectly fine.
When to rehydrate, and when to skip it
What hasn't changed is the physics. The higher the gravity of the wort, the harder it is for dry yeast to rehydrate cleanly. A cell coming out of dormancy has to take on water and rebuild its membrane before it can control what crosses that membrane, and a sugar-heavy wort works against it. The sugars push their way in before the cell is ready to keep them out. In a low-gravity beer there is little pressure to fight, so it barely matters. Brew a 3% mild and I would not give rehydration a second thought. Brew an 8% double IPA and you really should rehydrate. My hard line is gravity: above 1.065 (16°P), always rehydrate.
It is worth being clear about the trade, though. Even where it is not strictly necessary, proper rehydration always leaves you with more live, viable yeast than sprinkling dry does, at any gravity. Rehydrating only works against you when you do it badly: water that is too hot or too cold, yeast left sitting in the water for hours before it goes in, dirty water that contaminates the culture, or heavily chlorinated water that damages the cells. Avoid those and you cannot lose by rehydrating.
The process
Start with the water. Use tap or brewing water with the chlorine removed, and never use distilled or reverse-osmosis water. It has no mineral salts, and that absence is deadly to rehydrating yeast, the same osmotic problem that makes a big beer hard on them. If your yeast's manufacturer prints its own temperatures, volumes, or timing, follow those. The steps below work well for the vast majority of yeasts when you do not have specific guidance.
- Sanitize your scissors and the spot on the pack you plan to cut, using 70% alcohol.
- Measure out water at ten times the weight of the yeast: 10 grams of yeast wants 100 ml of clean, ideally sterile, water.
- Bring the water to 86–95°F (30–35°C).
- Sprinkle the yeast across the surface, as spread out as you can manage. A single clump leaves most of the cells out of contact with the water.
- Leave it undisturbed for 15 minutes. Do not stir yet. Stirring now can tear the rehydrating membranes. After 15 minutes, give it a gentle stir to mix everything together.
- Wait another 5 minutes, then bring the slurry down toward your wort temperature slowly, no more than a 18°F (10°C) drop every 5 minutes. The easy way is to add a little of the wort you are about to pitch into. Say the yeast is at 90°F (32°C) and the wort at 68°F (20°C), a 22°F (12°C) gap: add a volume of wort equal to the yeast and water you already have, and you will land roughly halfway between. Rest 5 minutes after that, and pitch.
Two things people get wrong at the end. First, do not dawdle: cool and pitch without long delays. Yeast left sitting in water starts spending the reserves it needs for a strong start, and you give back the viability and vitality you were trying to protect. Second, do not skip the cooling. Dropping warm, rehydrated yeast straight into much colder wort is a big, fast temperature swing, and that shock stresses the cells and can hurt the fermentation that follows.
Tough fermentations
For the hardest jobs, very high-gravity worts especially, I reach for a rehydration nutrient. Lallemand's GO-FERM PROTECT® is the one I use; it puts vitamins, minerals, and sterols into the rehydration water so the cells come back stronger and survive conditions that would otherwise thin the herd. I use 1.25 grams of it per gram of dry yeast, stirred into the rehydration water before the yeast goes in. It is cheap insurance when you are asking a lot of the pitch.
Once the yeast is healthy and in the wort, the next question is simply how much to pitch. The Yeast Pitching Rate Calculator works that out, and the guide to making starters covers the times you need to grow a pitch rather than rehydrate one.